1. Field of the Invention
The present invention relates to a focus detecting device for use in a camera and, more particularly, to an improvement in the arrangement of a focus detecting device of a type which utilizes detection of a contrast between neighboring sections of an image.
2. Description of the Prior Art
A prior art focus detecting device is shown in FIG. 1a which includes an objective lens 2 and a biconcave cylindrical lens 6 having a curvature only in up and down directions and no curvature in a direction perpendicular to a plane of drawing. The objective lens 2 converges light beams reflected from a target object 4 to be focused, and the cylindrical lens 6 diverges the light beams up and down to form a stretched line image on a light receiving unit 8.
The light receiving unit 8, as best shown in FIG. 1b, includes a substrate 10, a first array 12 of a plurality of light sensitive elements aligned in a row with a predetermined pitch and deposited on a front face of the substrate 10 facing the cylindrical lens 6, and a second array 14 of a plurality of light receiving elements aligned similarly on the front face of the substrate 10. Each light sensitive element has an identical rectangular configuration. The first and second arrays 12 and 14 are aligned parallel to each other above and below a center line of the substrate 10, which center line locates perpendicularly to an optical axis 3 of the lenses 2 and 6. Both of arrays 12 and 14 are positioned closely adjacent to each other. Neighboring light sensitive elements in each of the first and the second arrays 12 and 14 are also positioned closely to each other.
A transparent plate 15 is deposited so as to cover only light sensitive elements of the second array 14. Therefore, the optical path length from the cylindrical lens 6 to the second array 14 is longer than the optical path length from the cylindrical lens 6 to the first array 12. Therefore, the focus condition of an image of the target object 4, formed on the first array 12 by the objective lens 2 and the cylindrical lens 6, is different from that formed on the second array 14. More particularly, when the lens 2 is so positioned as to properly focus the image of the target object 4 on a predetermined focal plane FP1 positioned on optically middle plane of the first and second arrays, the image of the target object 4 on the first array 12 will be out-of-focus such that other object further away from the lens 2 than the target object 4 will be properly focused on the first array 12 and, at the same time, the image of the target object 4 on the second array 14 will be out-of-focus such that further other object which is closer to lens 2 than the target object 4 will be properly focused on the second array 14. The focus conditions of the images each formed on the first or the second array 12 or 14 are respectively determined by means of detecting contrast of each image in the direction perpendicular to the drawing in FIG. 1a. Such contrast can be detected from the differences of signals generated by neighboring light sensitive elements of the first or the second array 12 or 14.
The cylindrical lens 6 stretches the image in such a manner that a point image to be formed on the light receiving unit 8 by the objective lens 2 will be expanded in a certain direction to convert the point image into a line image. The direction of such a stretch (hereinafter referred to as a stretch direction) is so adjusted as to be in a perpendicular relation to the direction of alignment of the first, and second, array (hereinafter referred to as an array direction).
More particularly, when there is no cylindrical lens 6, a center point 4a of the target object 4 will be focused at a center 10a (FIG. 2) on the predetermined focal plane FP. When the cylindrical lens 6 is provided, however, the point image 10a will be stretched and converted into a line image extending between points 10a1 and 10a2 (FIG. 2) covering both the first and second arrays 12 and 14. Therefore, the first and second arrays 12 and 14 receive approximately the same amount of light reflected from the point 4a.
According to the prior art focus detecting device, however, there is such a disadvantage that the first and second arrays 12 and 14 receive different amount of light when a whole image of the target object 4 is taken into consideration. For example, a point 4b on the target object 4 is focused on the focal plane FP1 as a line image extending between points 10b1 and 10b2 (FIG. 2) covering almost only the second array 14. And, a point 4c on the target object 4 is focused on the focal plane FP1 as a line image extending between points 10c1 and 10c2 (FIG. 2) covering almost only the first array 12. Thus, the first array 12 almost only receives the light beams reflected from the lower half portion of the target object 4, and the second array 14 almost only receives the light beams reflected from the upper half portion of the target object 4 and, therefore, the total light intensity of the image formed on the first array 12 differs from that on the second array 14. Such a difference is eminent particularly when the brightness difference of the target object 4 between its upper portion and lower portion is great. Therefore, the difference of the focus condition between the images on the first and second arrays 12 and 14 can not be properly evaluated by using the above light receiving unit 8.
In order to eliminate such a disadvantage, one approach is to arrange the first and second arrays 12 and 14 as close as possible to the optical axis 3, that is, to shorten the length of each light sensitive element in the stretch direction and, at the same time, to narrow the distance between the first and second arrays 12 and 14. When such an arrangement is employed, however, a light receiving area of each light sensitive element becomes small and, therefore, an electric signal produced from each light sensitive element becomes weak. Thus, in the processing circuit, it is very difficult to distinguish the wanted signal from the noise signal.
It is to be noted that the light receiving area of a light sensitive element is defined by its length in the stretch direction and its width in the array direction. Therefore, in the above arrangement, it may be possible to obtain a light sensitive element having a large area by increasing the width of the light sensitive element in its array direction. However, since the detectible spatial frequency of the image by the light sensitive elements becomes low in proportion to increase the width thereof, the increase of the width of the light sensitive element results in poor detection of the focus condition.
Another approach is to shift the position of the cylindrical lens 6 towards objective lens 2 to elongate the line image in the stretch direction so that the line images each of points 4b and 4c cover both arrays 12 and 14. If this arrangement is employed, it is necessary to enlarge the size of the cylindrical lens 6 particularly in the stretch direction. Since a single-reflex-lens camera, which utilizes its picture-taking-lens also for the objective lens 2, has a limited space, such a large cylindrical lens 6 with a considerably long distance between the cylindrical lens 6 and the light receiving unit 8 is not suitable. Furthermore, when the light receiving unit 8 and the cylindrical lens 6 are prepared together as a module, the size of such a module becomes undesirably large.